This invention relates to a sorting apparatus.
Most conventional apparatuses for sorting particulate material have a conveyor belt which causes the particles to move successively, usually in free flight, through an excitation zone, a detection zone and an ejection zone. To enable the apparatus to handle a large throughput, the particles are arranged in a wide monolayer on the belt, with a number of particles across the width of the belt.
In the excitation zone, the particles may, for instance, be subjected to X-radiation in a case where diamonds are to be sorted from gangue. Downstream of the excitation zone is a detection zone where a series of photomultiplier (PM) tubes are arranged across the width of the belt to detect luminescence in the particles. Further downstream, a bank of air blast ejectors is arranged. When a luminescing particle is detected at the detection zone, the appropriate ejector is actuated to issue an air blast which deflects the relevant particle out of the main stream.
To ensure that accurate detection and ejection takes place, it is essential that each PM tube and each ejector cover a small area only of the width of the falling stream of particles. If, for instance, the area "seen" by a PM tube is large, an "eject" signal could be issued when any one of a number of particles "seen" by the PM tube is luminescing. Similarly, if the ejector blasts too large an area, non-luminescing particles could also be ejected. With continuing research, the sizes of the ejectors have been decreased with the result that they are able these days to cover smaller areas than was hitherto possible. However the cost of many small PM tubes has proved to be prohibitive, and the present invention seeks to provide an alternative solution to the detection problem.